1. Field of the Invention
This invention relates generally to integrated circuits, and, more particularly, to a two-stage voltage pump for providing an internal voltage supply at a higher voltage magnitude than an external power supply voltage.
2. Description of the Related Art
System designs are routinely constrained by a limited number of readily available power supply voltages (V.sub.cc). For example, consider a portable computer system powered by a conventional battery having a limited power supply voltage. For proper operation, different components of the system, such as display, processor, and memory components employ diverse technologies that require power to be supplied at various operating voltages. Components often require operating voltages of a greater magnitude than the power supply voltage and, in other cases, a voltage of reverse polarity. The design of a system, therefore, includes power conversion circuitry to efficiently develop the required operating voltages. One such power conversion circuit is known as a voltage pump. The demand for highly-efficient and reliable voltage pump circuits has increased with the increasing number of applications utilizing battery powered systems, such as notebook computers, portable telephones, security devices, battery-backed data storage devices, remote controls, instrumentation, and patient monitors, to name a few.
Inefficiencies in conventional voltage pumps lead to reduced system capability and lower system performance in both battery and non-battery operated systems. Inefficiency can adversely affect system capabilities, e.g., limited battery life, excess heat generation, and high operating costs. Samples of lower system performance include low speed operation, excessive operating delays, loss of data, limited communication range, and inability to operate over wide variations in ambient conditions including ambient light level and temperature.
In addition to constraints on the number of power supply voltages available for system design, there is an increasing demand for reducing magnitudes of the power supply voltages. Current voltage pump circuits have difficulty providing sufficiently high output voltages as the supply voltage approaches two volts.
An exemplary prior art voltage pump circuit 10 is shown in FIG. 1. Only one of the two complimentary phases is shown. The other phase is symmetrical to the phase shown in the voltage pump circuit 10. Typically, an oscillator signal is used to generate non-overlapping complimentary enable signals for alternately firing each phase. The voltage pump circuit 10 includes capacitors 12, 14. During a charging phase, the output signals of inverters 16, 18 are held at a low voltage level. The charging phase of one phase corresponds to the firing phase of the other phase. During the charging phase, control signals C1, C2 enable the gates of transistors 20, 22, coupling the capacitors 12, 14 to a voltage supply 24 (Vdd) and charging them to a voltage of Vdd. The control signals C1, C2 are generated from the firing signals of the other phase.
During the firing phase, the transistors 20, 22 are disabled, and the inverters 16, 18 are driven to a high output voltage (ie., booted), thus raising the voltage on the capacitors 12, 14 to .about.2*Vdd. The inverter 18 is enabled slightly after the inverter 16 to allow the capacitor 12 to fully boot. After the inverter 18 is enabled, the voltage present on the capacitor 14 enables a pass gate 24, which connects the capacitor 12 to the output voltage line, Vccp, and the capacitor delivers its charge. Clamping transistors 26, 28 keep the voltage on the capacitors 12, 14 from falling below a threshold drop, Vt, under Vdd during the charging phase to increase circuit efficiency. A filtering capacitor 30 is coupled to the Vccp output terminal. The voltage pump circuit 10 has an unregulated output voltage of about 3.5V (Vccp) with a 2V supply voltage (Vdd). This output voltage is insufficient for some integrated circuit applications.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.